Immersive Web SDK: Plane asset — primitive or procedural

I’m adding a floor plane to my scene.  I can do it easily by adding Three.js code to `index.ts`, but is there a way to add it as an asset? The IWSDK Scene Editor has primitive-box, primitive-capsule, primitive-cylinder and primitive-sphere, but no other Three.js geometries.   

Can I add a new primitive?  Or would this be a procedural asset?  Either way, how does one actually do that? There’s no documentation of either, and the LLMs are stumped.

https://iwsdk.dev/api/core/classes/RenderableAssetRegistry.html#returns-7